Optical beam steering is often required where information from an optical beam must be relayed from one location to another. High data rate, secure laser communications, target designation, and laser radar are a few of the applications in which optical beam steering is required. Optical beam steering may be provided by a single aperture, which directs a light beam in the desired direction.
Devices for single aperture beam steering are well known in the art. Single aperture beam steering can be implemented with electromechanical systems. Such systems generally consist of a mirror mounted on an electrical actuator. These systems provide relatively low losses for the strength of the reflected beam. However, such electromechanical systems are limited to low response frequencies, up to the order of 1 KHz. The moving parts of an electromechanical system along with size and weight factors are considered to be major limitations of such a system.
Multiple aperture beam steering devices may be provided by compact arrays of non-mechanical beam deflectors, such as optical micro electromechanical systems arrays (O-MEMS) or liquid crystal arrays. The optical signal provided to these devices is generally split into multiple optical signals. The array then actually consists of multiple optical apertures that act to steer and radiate multiple optical signals in a desired direction. However, the application of multiple aperture steering devices for fast optical communications and radar applications requires precise synchronization of the optical signals at the individual apertures for different pointing angles of the device. This is necessary to avoid signal scrambling due to mixing of non-synchronized outputs emerging from individual emitters.
Time synchronization is also required since the multiple apertures are generally deployed in a relatively flat plane. Thus, when an optical signal is steered to an angle other than exactly perpendicular to that plane, unsynchronized outputs from the individual apertures do not arrive at a receive point at the same time. This problem is particularly noticeable when the optical signal comprises pulsed signals. In this case, the optical pulse received from the radiating element furthest from the receive point will lag the pulse received from the closest radiating element. Performance of the optical transmitting system is improved when the individual optical beams are made time-coincident to create a time-coincident optical beam.
Further performance enhancement of the optical transmitting system can be provided by improving the spatial coherence of the wavefront produced by the multiple aperture beam steerer, also known in microwave applications as the phase array antenna. Multiple aperture beam steerers such as phased array antenna systems employ a plurality of individual antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. In a phased array, the relative phases of the signals provided to the individual elements of the array are controlled to produce an equiphase beam front in the desired pointing direction of the antenna beam. The transmitted electromagnetic wave, at a given element, is represented by an envelope and fast oscillations of the electric field of the optical carrier. Time synchronization is the alignment of the transmissions of the envelopes, whereas spatial coherence is the alignment of the transmissions of the fast oscillations of the electric fields.
Time synchronization is needed to create a time-coincident optical beam. Spatial coherence is needed so that the transmitted electromagnetic waves from each element combine to form a flat wavefront. The flat wavefront allows for the efficient focusing of the electromagnetic waves onto a target, e.g., into a fiber or onto a small detector. If the system has poor spatial coherence, i.e. the phases are not aligned, the beam can not be focused into the smallest possible spot.
Additionally, phase alignment allows for beam steering. For beam steering the phases are preferably aligned in such a way that they form a tilted plane with respect to the optical steerer (or the surface of the phase array antenna). Further, adjustment of individual phases of the optical signal at a given element allows for the compensation of atmospheric aberrations.
True-time delay for optical signal transmission may be achieved by purely electronic means by splitting an information carrying electronics signal into a number of channels equal to the number of optical apertures. The delay required for optical beam steering is then applied to each one of the channels separately. The properly delayed signals then drive electro-optic modulators that control outputs of the corresponding apertures. This electronic approach to true-time delay for optical beam steering requires very sophisticated and fast electronics that increase cost and complexity.
Optical control systems for producing selected time delays in signals for microwave phased array antennas are well known in the art. Different types of optical architectures have been proposed to process optical signals to generate selected delays, such as routing the optical signals through optical fiber segments of different lengths; using deformable mirrors to physically change the distance light travels along a reflected path before transmission; and utilizing free space propagation based delay lines, the architecture of which typically incorporates polarizing beam splitters and prisms. These techniques may also be used for providing the true-time delays required for optical beam steering. However, these techniques are also costly and complex.
A true-time delay feeder for microwave phased array antenna has been proposed by Corral et al. in “Continuously Variable Time Delay Feeder for Phased Array Antenna Employing Chirped Fiber Gratings”, IEEE Trans. Microwave Theory and Tech., vol. 45(8), 1997, p. 1531, and in “True Time-Delay Scheme for Feeding Optically Controlled Phased-Array Antennas Using Chirped Fiber Gratings”, IEEE Phot. Tech. Lett., vol. 9(11), 1997, p. 1529. In the system described by Corral et al., each element of the microwave antenna is fed by an individually-tunable optical carrier modulated by the microwave signal. The carrier passes though a dispersive element, a chirped fiber grating, which introduces a delay. The delay for each antenna element is controlled by tuning the corresponding optical carrier. However, the true time-delay feeder described by Corral et al. requires a large number of independently tunable sources (equal to the number of elements in the array). Moreover, for some applications, just as many modulators may be required. Thus, an optical beam steerer according to the teachings of Corral et al. amounts to a complicated and cumbersome system. Further, the teachings of Corral et al. allow only for time synchronization and phase tuning of microwave signals but not for optical signals.
Another true-time delay feeder for phased array antennas has been described in the commonly assigned U.S. patent application Ser. No. 09/877,976, filed Jun. 8, 2001, in the name of Pepper, herein incorporated by reference. In this true-time delay feeder liquid crystals provide both true-time delay and spatial coherence.
Another true-time delay feeder for phased array antenna has been described in commonly assigned, U.S. patent application Ser. No. 09/738,584, filed Dec. 16, 2000, in the name of Ionov, herein incorporated by reference. This true-time delay feeder addresses the issue of temporal overlap of the optical signals leaving the individual apertures. In addition, the optical signals for the individual apertures originate from different spectral components of the original beam. The teachings of Ionov relate to the technical problem of time synchronization.
A fast electro-optic delay generator has been described in commonly assigned U.S. patent application Ser. No. 09/545,632, filed Apr. 7, 2000, in the name of Ionov, herein incorporated by reference. The fast electro-optic delay generator addresses the technical problem of time synchronization. The fast electro-optic delay generator is a costly and complicated device that offers very high speed operation, which may not be necessary for the given application.
A sequential true-time delay system has been described in R.L.Q. Li, Fu, R. Chen, “High Density broadband true-time delay unit on a single substrate,” SPIE Vol. 3006 (1997) pp. 256-263. This approach results in non-uniform beam “fan-out” intensity, where the output power decreases with each subsequent optical tap as the beam propagates down the sequential true-time delay scheme.
Hence, a need exists in the art for a true-time delay generator for multiple aperture optical beam steering that addresses the issues of temporal and spatial coherence over the entire field-of-view of the full composite aperture.